摘要:

The torsional composite oscillator enables the mechanical damping, the strain amplitude, and the shear modulus of a specimen to be determined quickly and accurately, making it a powerful technique for measuring transient phenomena. In this paper the equivalent circuit for the composite oscillator is derived, and the resulting formulas for the mechanical damping and strain amplitude are confirmed by various alternative methods of calibration. Experimental arrangements and procedures are described which indicate how this technique can be used for strain amplitudes ranging from 10−12to 10−3.

ISSN:0021-8979    

DOI:10.1063/1.1663533

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

A new mechanism is proposed to explain the formation of growth spirals in controlled melt‐grown crystalline samples of Ba4SrNb10O30. Several distinguishing features of these spirals cannot be explained on the basis of the classical core‐first growth theory. These features are examined in detail, especially in comparison with the inherent or implied features of the classical spiral model. Characteristics of the spirals are predicted based on solidification theory and are found to agree with the observed results.

ISSN:0021-8979    

DOI:10.1063/1.1663534

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

Results are presented for the Raman spectra, the far‐infrared reflectivity spectra, and the radio‐frequency dielectric constant of several selected PLZT ceramics. Vibrational modes are assigned by comparing these results with previous Raman and infrared measurements on single‐crystal PbTiO3, and the PZT and PLT mixed ceramic systems. The mode responsible for the temperature variation of the low‐frequency dielectric constant was not observed but is shown to be below 65 cm−1.

ISSN:0021-8979    

DOI:10.1063/1.1663535

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

The nonlinear interaction of surface acoustic waves propagating in the same direction has been treated from the rigorous theory of thermoelasticity. Exact expressions were derived for the nonlinear volume forces and surface stresses, and the nonlinear cross sections were calculated under two separate assumptions. First, the work done by the nonlinear forces and surface stresses on a normal‐mode surface wave was evaluated and assumed to be totally converted into surface‐wave energy. In the second approach, a linear analytic solution was formulated which simultaneously satisfies the nonlinear wave equation and maintains a stress‐free boundary condition. Numerical values for the strength of second‐harmonic generation were calculated for 24 materials. The characteristics of nonlinear surface‐wave generation were compared with those of nonlinear bulk‐wave generation and significant differences were found. Coupled amplitude equations were derived to treat the case of multiharmonic generation. Finally, a characteristic power parameter which describes energy conversion from the fundamental into the second harmonic was derived for surface waves and evaluated for 24 materials.

ISSN:0021-8979    

DOI:10.1063/1.1663536

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

In this paper some new observations on electrically induced damage in NaCl single crystals are presented. Microscopic studies made on surfaces normal and parallel to the applied electric field and inside the sample allowed us to determine the crystallographic and geometric characteristics of the breakdown channel, the nature of the different damaged zones around it, and their manifestations on the external surfaces of the sample. A mechanism based on the model of Bullough and Gilman is proposed to explain the experimental results. It is suggested that as a high amount of energy is transferred to the solid in a very short time compared with the elastic relaxation time, an interior implosion can be generated. An implosion of this kind would originate high‐intensity shock waves, which would be responsible for the observed damage by causing intense heating, sublimation and decomposition of the material, spallation, and cleavage.

ISSN:0021-8979    

DOI:10.1063/1.1663537

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

To understand the processes which control the rate of plastic deformation it is necessary to determine the atomic parameters and in particular the activation volume. The true activation volume can be identified only if the stress dependence of deformation rate is described by a valid expression. To determine this expression a kinetics analysis has to be carried out, often for a system of parallel and consecutive energy barriers. This is an intricate procedure which has to be carried out in well‐defined steps and in the order of increasing complexity of barrier combinations. Furthermore, the validity of the numerical values obtained in the analysis has to be established. This process of deformation kinetics analyis is elucidated and the method is described fully.

ISSN:0021-8979    

DOI:10.1063/1.1663538

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

The atomic processes associated with the plastic deformation of commercial‐quality sintered iron were investigated. Stress relaxation tests were carried out over the 17–19‐kg mm−2range of initial stress levels at room temperature. The experimental results were analyzed with the deformation kinetics theory discussed in Paper I. The analysis showed that the rate‐controlling mechanism is associated with a system of two consecutive energy barriers. At high stress levels only the first barrier is effective. The corresponding activation volume of ∼ 200 b3, measured in these tests, suggested that the Peierls‐Nabarro mechanism was rate controlling at this level. It is proposed that the second barrier is either the second half of the ``camel‐hump'' Peierls‐Nabarro barrier or the resultant of a series of dragging‐point barriers controlling the spreading of the double kink.

ISSN:0021-8979    

DOI:10.1063/1.1663539

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

The intensity of neutrons diffracted from ADP crystals is enhanced when the crystals are vibrated in their fundamental piezoelectric modes. The neutrons are also modulated at a frequency approximately twice that of the crystal frequency. These effects can be explained by the dynamical theory of diffraction and are similar to the effects of neutrons diffracted by piezoelectric quartz crystals.

ISSN:0021-8979    

DOI:10.1063/1.1663540

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

This paper reports results obtained through computer simulation of the thermally activated glide of an idealized dislocation through a random array of point obstacles. The array size is fixed at 999, and the dislocation‐obstacle interaction is taken to have a simple step form. The variables governing glide are then the resolved shear stress, the temperature, and the obstacle strength, which may be phrased as dimensionless parameters. The principal subjects studied are (1) the stress required for athermal glide at a given obstacle strength and the characteristics of the obstacle configurations which determine this stress, (2) the glide path taken by the configuration and the characteristics of the obstacle configurations encountered along this glide path, (3) the velocity of glide and its dependence on the stress, the temperature, and the obstacle strength.

ISSN:0021-8979    

DOI:10.1063/1.1663541

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

Elliott's model of a straight‐fronted cleavage crack is used to conduct an analytical Peierls‐Nabarro‐type calculation appropriate for a two‐dimensional lattice where the atomic spacings are respectivelyaandbnormal and parallel to the cleavage crack. The analysis enables the effect of the discreteness of the atomic structure on brittle cleavage crack extension to be assessed and circumvents some of the limitations in a previous analysis due to Thomson, Hsieh, and Rana. The general conclusion emerging from the present work is that the magnitude of the discreteness effect, reflected in the ratio between the crack extension stress and that predicted by the continuum‐based Griffith relation, is greater the smaller the crack front width which thereby substantiates Thomson, Hsieh, and Rana's inferences. Proceeding from this conclusion, the wider problem of relating the magnitude of the discreteness effect, crack front width, and the type of atomic bonding is discussed from an essentially physical viewpoint. As a result of such considerations it is suggested that, in general, the discreteness effect will be small and the crack front will be wide with a material that is more resistant to elastic shear than tension, as in a covalently bonded solid where the bonding is directional. Since dislocation mobility is difficult in such a material, it would therefore seem that existing arguments regarding brittle cleavage crack extension should not be significantly affected by the introduction of discreteness considerations and that very special circumstances are likely to be required to give a large discreteness effect, as would be reflected in a brittle crack extension stress markedly in excess of that predicted by the Griffith relation.

ISSN:0021-8979    

DOI:10.1063/1.1663542

出版商:AIP    

年代:1974

数据来源: AIP